<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title> UDT Reference</title>
<link rel="stylesheet" href="udtdoc.css" type="text/css" />
<style type="text/css">
<!--
.style1 {color: #FF0000}
-->
</style>
</head>

<body>
<div class="ref_head">&nbsp;UDT Reference: Functions</div>

<h4 class="func_name"><strong>select</strong></h4>
<p>The <b>select</b> method queries one or more groups of UDT sockets. </p>
<p class="style1">Note: select is deprecated. Please use <a href="epoll.htm"><strong>epoll</strong></a> instead, which is far more efficienct. </p>
<div class="code">int select(<br />
&nbsp; int <font color="#FFFFFF">nfds</font>,<br />
&nbsp; UDSET* <font color="#FFFFFF">readfds</font>,<br />
&nbsp; UDSET* <font color="#FFFFFF">writefds</font>,<br />
&nbsp; UDSET* <font color="#FFFFFF">exceptfds</font>,<br />
&nbsp; const struct timeval* <font color="#FFFFFF">timeout</font><br />
);</div>

<h5>Parameters</h5>
<dl>
  <dt><i>nfds</i></dt>
  <dd>[in] Ignored. For compatibility only.</dd>
  <dt><em>readfds</em></dt>
  <dd>[in, out] Optional pointer to a set of sockets to be checked for readability.</dd>
  <dt><em>writefds</em></dt>
  <dd>[in, out] Optional pointer to a set of sockets to be checked for writability.</dd>
  <dt><em>exceptfds</em></dt>
  <dd>[in, out] Ignored. For compatibility only.</dd>
  <dt><em>timeout</em></dt>
  <dd>[in] The future time when this call should be timeout.</dd>
</dl>

<h5>Return Value</h5>
<p>If any of the read or write query is positive, <strong>select</strong> returns the number of UDT sockets that are read for read/write. If no socket is ready before timeout, zero is 
returned. If there is any error, UDT::ERROR is returned and the specific error information can be retrieved using <a href="error.htm">getlasterror</a>. The <i>readfds</i> and/or 
<i>writefds</i> will be updated to contain the ready sockets only.</p>


<table width="100%" border="1" cellpadding="2" cellspacing="0" bordercolor="#CCCCCC">
  <tr>
    <td width="17%" class="table_headline"><strong>Error Name</strong></td>
    <td width="17%" class="table_headline"><strong>Error Code</strong></td>
    <td width="83%" class="table_headline"><strong>Comment</strong></td>
  </tr>
  <tr>
    <td>EINVPARAM</td>
    <td>5003</td>
    <td>All three socket sets are empty or at least one of the socket is invalid.</td>
  </tr>
</table>

<h5>Description</h5>
<p>The UDSET is a structure to store the UDT socket descriptors. If should only be processed with the following macros.</p>
<dl>
  <dt><b>UD_CLR</b>(<i>u, *set</i>)</dt>
  <dd>Removes the descriptor <i>u</i> from <i>set</i>.</dd>
  <dt><b>UD_ISSET</b>(<i>u, *set</i>)</dt>
  <dd>Nonzero if <i>u</i> is a member of <i>set</i>; otherwise zero.</dd>
  <dt><b>UD_SET</b>(<i>u, *set</i>)</dt>
  <dd>Add descriptor <i>u</i> to <i>set</i>.</dd>
  <dt><b>UD_ZERO</b>(<i>*set</i>)</dt>
  <dd>Initialize <i>set</i> to an empty set.</dd>
</dl>

<p>The UDT descriptors sets originaly contains the sockets whose status is to be queried. When <strong>select</strong> returns, the descriptors sets only contain the sockets that are 
ready for IO. UD_ISSET can be used to check which one is ready.</p>
<p><i>readfds</i> is used to detect if any socket in this set is available for reading (recv, recvmsg), for accepting a new connection (accept), or the associated connection is broken. 
<i>writefds</i> is used to detect if any socket in this set has available buffer for sending (send, sendmsg). Currently <i>exceptfds</i> is not used.</p>

<h5>Example</h5>
<p>The following example shows how to check if a UDT socket is available for <strong>recv</strong>.</p>

<div class="code">
<p>
UDTSOCKET u;<br>
...<br>
<br>
timeval tv;<br>
UDSET readfds;<br>
<br>
tv.tv_sec = 1;<br>
tv.tv_usec = 0;<br>
<br>
UD_ZERO(&readfds);<br>
UD_SET(u, &readfds);<br>
<br>
int res = UDT::select(0, &readfds, NULL, NULL, &tv);<br>
<br>
if ((res != UDT::ERROR) && (UD_ISSET(u, &readfds)))<br>
&nbsp;&nbsp;&nbsp;// read data from u.<br>
else<br>
&nbsp;&nbsp;&nbsp;// timeout or error<br>
</p>
</div>


<h5>See Also</h5>
<p><strong><a href="selectex.htm">selectEx</a></strong>, <a href="epoll.htm"><strong>epoll</strong></a> </p>
<p>&nbsp;</p>

</body>
</html>
